The accurate description of micro-/nanopores in deep coal reservoirs plays an important role in evaluating the reservoir properties and gas production capacity of coalbed methane (CBM). This work studies nine continuous samples of high–rank coal from the Daning–Jixian area of the Ordos Basin. Maceral analysis, proximate analysis, field emission scanning electron microscopy (FE-SEM), low-pressure CO₂ adsorption (LPA), low-temperature N₂ adsorption (LTA) and high-pressure methane adsorption (HPMA) experiments were conducted for each sample. The fractal dimension (D) of the LPA data was calculated by using the micropore fractal model. The characteristics of the deep coal reservoir pore structure, proximate analysis, relationship between maceral and fractal dimensions, and gas adsorption capacity of the micropores are discussed. The results showed that the combination of LPA with nonlocalized density functional theory (NLDFT) models and LTA with NLDFT models can more accurately determine the pore size distribution of the micropores. The pore volume (PV) and specific surface area (SSA) of the coals were distributed in the ranges of 0.059~0.086 cm³/g and 204.38~282.42 m²/g, respectively. Although the degree of micropore development varies greatly among different coal samples, the pore distribution characteristics are basically the same, and the PV and SSA are the most developed in the pore size range of 0.4–0.7 nm. Ash content (A_d) and mineral composition are two major factors affecting micropore structure, but they have different impacts on the fractal dimension. The higher the vitrinite content, moisture content (M_ad) and Ad are, the larger the micropore fractal dimension (D) and the stronger the heterogeneity of the pore structure. Micropores account for 99% of the total SSA in coal, and most methane can be adsorbed in micropores. The fractal dimension of micropores can be used to evaluate the pore structure characteristics. The larger the fractal dimension, the smaller the micro-SSA and micro-PV of the coal sample. Fractal analysis is helpful to better understand the pore structure and adsorption capacity of CBM reservoirs.

Shale and coal in the transitional marine–continental facies of the Ordos Basin serve as unconventional natural gas reservoirs, with their pore structures controlling gas adsorption characteristics and occurrence states. To quantitatively characterize the pore structure features and differences between these two reservoirs, this study takes the Shanxi Formation shale and coal in the Daning–Jixian area on the eastern margin of the Ordos Basin as examples. Field-emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion, low-temperature N₂ adsorption, and low-pressure CO₂ adsorption experiments were employed to analyze and compare the full-scale pore structures of the shale and coal reservoirs. Combined with methane isothermal adsorption experiments, the gas adsorption capacity and its differences in these reservoirs were investigated. The results indicate that the average total organic carbon (TOC) content of shale is 2.66%, with well-developed organic pores, inorganic pores, and microfractures. Organic pores are the most common, typically occurring densely and in clusters. The average TOC content of coal is 74.22%, with organic gas pores being the dominant pore type, significantly larger in diameter than those in transitional marine–continental facies shale and marine shale. In coal, micropores contribute the most to pore volume, while mesopores and macropores contribute less. In shale, mesopores dominate, followed by micropores, with macropores being underdeveloped. Both coal and shale exhibit a high SSA primarily contributed by micropores, with organic matter serving as the material basis for micropore development. The methane adsorption capacity of coal is 8–29 times higher than that of shale. Coal contains abundant organic micropores, providing a large SSA and numerous adsorption sites for methane, facilitating gas adsorption and storage. This study comprehensively reveals the similarities and differences in pore structures between transitional marine–continental facies shale and coal reservoirs in the Ordos Basin at the microscale, providing a scientific basis for the precise evaluation and development of unconventional oil and gas resources.

To study the microscopic pore characteristics of marine–continental transitional shale, we studied the Daning–Jixian block of the Shanxi Formation using low-pressure CO₂ adsorption (LP-CO₂A) and low-temperature N₂ adsorption (LT-N₂A) methods in conjunction with field emission scanning electron microscopy (FE-SEM), geochemistry, and mineral composition analysis in order to obtain pore structure characteristic parameters. The fractal dimension of the pores was calculated using the Frankel–Halsey–Hill (FHH) model, and the study also discusses the factors that influence the pore structure. The study found that the marine–continental transitional phase shale of the Shanxi Formation has clay mineral contents ranging from 36.24% to 65.21%. The total organic carbon (TOC) contents range from 0.64% to 9.70%. Additionally, the organic matter maturity is high. The FE-SEM and gas adsorption experiments revealed that the transitional shale of the Shanxi Formation possesses a diverse range of pore types with relatively large pore sizes. The dominant pore types are organic and intragranular pores, with pore morphologies predominantly appearing as slit and parallel plate structures. According to the experimental data on gas adsorption, the total SSA values range from 11.126 to 47.220 m²/g. The total PV values range from 0.014 to 0.056 cm³/g. Micropores make up a greater proportion of the total SSA, whereas mesoporous pores make up a greater proportion of the total PV. The distribution of shale pore fractal dimensions D₁ and D₂ (D₁ is 2.470 to 2.557; D₂ is 2.531 to 2.755), obtained through LT-N₂A data, is relatively concentrated. D1 and D2 have a positive correlation with the TOC content, clay mineral content, and BET-SSA, and D₁ and D₂ have a negative correlation with the quartz content. D₂ is positively correlated with the Langmuir volume, showing that D₂ can be used to evaluate the methane adsorption capacity.

Accurate characterization of coal reservoir micro- and nanopores is crucial in evaluating coalbed methane storage and gas production capacity. In this work, 12 coal-bearing rock samples from the Jurassic Yan’an Formation, Longdong area, Ordos Basin were taken as research objects, and micro- and nanopore structures were characterized via scanning electron microscopy, high-pressure mercury pressure, low-temperature N₂ adsorption and low-pressure CO₂ adsorption experiments. The main factors controlling coal pore structure development and the influence of pore development on the gas content were studied by combining the reflectivity of specular samples from the research area, the pore microscopic composition and the pore gas content determined through industrial analyses and isothermal absorption experiments. The results show that the coal strata of the Yan’an coal mine are a very important gas source, and that the coal strata of the Yan’an Formation in the study area exhibit remarkable organic and clay mineral pore development accompanied by clear microfractures and clay mineral interlayer joints, which together optimize the coal gas storage conditions and form efficient microseepage pathways for gas. Coalstone, carbonaceous mudstone and mudstone show differential distributions in pore volume and specific surface area. The general trend is that coal rock is the best, carbonaceous mudstone is the second best, and mudstone is the weakest. The coal samples’ microporous properties are positively correlated with the coal sample composition for the specular group, whereas there is no clear correlation for the inert group. An increase in the moisture content of the air-dried matrix promotes adsorption pore development, leading to increases in the microporous volume and specific surface area. CH₄ adsorption in coal rock increases with increasing pressure, and the average maximum adsorption is approximately 8.13 m³/t. The limit of the amount of methane adsorbed by the coal samples, V_L, is positively correlated with the pore volume and specific surface area, indicating that the larger the pore volume is, the greater the amount of gas that can be adsorbed by the coal samples, and the larger the specific surface area is, the greater the amount of methane that can be adsorbed by the coal samples. The P_L value, pore volume and specific surface area are not correlated, indicating that there is no direct mathematical relationship between them.

The complex geological environment in deep layers results in differences in the pore and fracture structures and states of coalbed methane (CBM) occurrences between deep and shallow coal reservoirs. The coexistence of multiphase gases endows deep CBM with both “conventional” and “unconventional” geological attributes. Based on systematically collected coal samples from the Benxi Formation in the Daning–Jixian area of the Ordos Basin, high-pressure mercury intrusion (HPMI), low-temperature N₂ adsorption (LTN₂A), and low-pressure CO₂ adsorption (LPCO₂A) experiments were conducted to characterise the pore structures across the full pore size distribution of the Benxi Formation coals. The aim of this research is to gain an in-depth understanding of the pore size distribution of full-size pores and to explore the factors influencing their pore structure and control over the gas content in coal reservoirs. The results indicate that the pore size distribution of the coal samples from the Benxi Formation in the study area is unimodal and that nanopores are present. The pore sizes are relatively small, with an average total pore volume (PV) of 0.073 cm³/g and an average total specific surface area (SSA) of 227.87 m²/g. Among these, micropores account for 92.26% of the total PV and 99.57% of the total SSA, making micropores the primary contributors to the gas storage space in the Benxi Formation coals. Mesopores and macropores contribute relatively little to the PV and SSA, which is unfavourable for CBM permeability. The development of pores in the Benxi Formation coals in the study area is influenced by the coal maturity, vitrinite content, and ash yield. Generally, the PV increases when the coal’s rank increases; an increase in the vitrinite content promotes the development of micropores, whereas a relatively high ash yield leads to decreases in the PV and SSA. The influence of the SSAs of coal pores on the gas content is reflected mainly by its effect on the adsorbed gas content. Since adsorbed gas molecules exist mainly in coal pores in the adsorbed state, the SSAs of coal pores strongly affect the storage capacity of coal for adsorbed gas.

Shale gas exploration and development in the Luzhou area continues to expand into deeper regions, yet a systematic understanding of the pore structure characteristics and gas-bearing properties of the deep shale reservoirs in this area remains lacking, which constrains the scientific extraction of deep shale gas in the region. Taking the deep shale of the Longmaxi Formation in the Luzhou block of the southern Sichuan Basin as the research object, comprehensive research methods such as FE–SEM, FIB–SEM three-dimensional reconstruction technique, LPCO₂A, LTN₂A, and methane isothermal adsorption experiments were employed to systematically study the pore structure characteristics and gas-bearing properties of the deep shale in the Longmaxi Formation. The research results show that the pores of the deep shale of the Longmaxi Formation are mainly composed of organic pores, intragranular dissolution pores, and interlayer pores of clay minerals, and organic pores are primarily isolated with poor connectivity. The PV of the shale is mainly contributed by mesopores, and the SSA is mainly contributed by micropores. The pore structure of the Long 1₁³ layer shale is the most developed. The D2 value is greater than the D1 value, indicating that the pore space structure of deep shale is more complex than its pore surface. The increase in TOC and quartz content appears to favor the development of pores and enhance reservoir homogeneity, while the increase in clay mineral content appears to inhibit pore formation and enhance heterogeneity. Micropores serve as the primary host for adsorbed gas, while mesopores serve as the primary host for free gas.